Nitride semi-conductive light emitting device is compact and configured to emit a light in UV-visible region with low electrical consumption, and have been widely studied for hygienic, medical, and industrial applications as well as development of illumination devices, precision machines, and the like. The nitride semi-conductive light emitting device has already been developed for practical use, in particular regions such as blue-color region.
As well as the nitride semi-conductive light emitting device emitting blue-color light (which is referred to as a blue-color light emitting diode, hereafter), the nitride semi-conductive light emitting device needs to have a further improved luminescence efficiency and light output. In particular, the semi-conductive light emitting device emitting a light in ultraviolet region (which is referred to as an ultraviolet light emitting diode (UV-LED), hereafter) provides a significantly lower light extraction efficiency and significantly smaller light output than the blue-color light emitting diode. The significantly small light extraction efficiency and light output have been obstacles to practical use of the light emitting diode in UV region. The significantly small light extraction efficiency and light output are presumably attributed to very low light emission efficiency (referred to as internal quantum efficiency, hereafter) in the light emitting layer. The internal quantum efficiency of the light emitting layer made of nitride semi-conductive material (nitride mixed crystal) is caused to be significantly lowered by transition, point defect, or the like which are highly formed in the light emitting layer. Especially, light emitting layer made of an AlGaN ternary mixed crystal including Al exhibits a considerably lowered internal quantum efficiency, for difficulty in growth of the crystal with high quality. AlGaInN quaternary mixed crystal obtained by an addition of In to AlGaN has been attracted as a mixed crystal less susceptible to the transition and the point defect, for application to the material of the light emitting layer. InAlGaN gives an improved internal quantum efficiency of the light emitting layer of the UV-LED, and has been studied for achieving practical use of the UV-LED. However, the UV-LED still gives too low internal quantum efficiency of the light emitting layer, and needs to be improved for achieving high external quantum efficiency of several tens of percent.
Japanese unexamined patent application publication No. 2007-73630 discloses a UV-light emitting diode comprising a single crystal substrate for epitaxial growth, an n-type nitride semi-conductive layer laminated to top side of the single crystal substrate with a first buffer layer interposed therebetween, a light emitting layer laminated to top side of the n-type nitride semi-conductive layer, and a p-type nitride semi-conductive layer laminated to top side of the light emitting layer. This UV-light emitting diode in Japanese unexamined patent application publication No. 2007-73630 comprises the light emitting layer having an AlGaInN quantum well structure, and a second buffer layer having the same composition as a barrier layer of the light emitting layer. The second buffer layer is provided between the n-type nitride semi-conductive layer and the light emitting layer. As being provided with the second buffer layer, the light emitting diode is found to give a high output of UV-light, compared to the light emitting diode not having the second buffer layer.
The nitride semi-conductive light emitting device such as this UV-light emitting diode in Japanese unexamined patent application publication No. 2007-73630 has a laminated structure, in which the second buffer layer is provided for suppressing the distortion of the light emitting layer resulting from the difference in lattice constant between the n-type nitride semi-conductive layer and the light emitting layer. However, this nitride semi-conductive light emitting device still fails to eliminate the problem related to an electric field caused by piezo effect (which is referred to as “piezoelectric field, hereafter) due to the difference in lattice constant between the well layer and the barrier layer which are formed of AlGaInN layers having different compositions. The piezoelectric field spatially separates each hole from each electron injected into the light emitting layer such that electron-accumulated portion is deviated from hole-accumulated portion in the thickness direction of the light emitting layer. It leads to decrease of probability in electron-hole recombination as well as decrease of the internal quantum efficiency of the light emitting layer, thereby causing reduction of the external quantum efficiency. This nitride semi-conductive light emitting device still has a poor planarity of a base for growth of the light emitting layer, although provided with the second buffer layer, thereby making it difficult to form a light emitting layer with a high quality.